[gdb/symtab] Remove superfluous handling of Ada main in write_cooked_index

I filed PR29179 about the following FAIL in test-case
gdb.ada/O2_float_param.exp with target board cc-with-gdb-index:
...
(gdb) break increment^M
Function "increment" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.ada/O2_float_param.exp: scenario=all: gdb_breakpoint: \
  set breakpoint at increment
...

The FAIL was a regression since commit 2cf349be0e ("Do not put linkage names
into .gdb_index").

Before that commit we had:
...
$ readelf -w foo > READELF
$ grep callee.*increment READELF
[1568] callee__increment: 5 [global, function]
[3115] callee.increment: 5 [global, function]
...
but after only:
...
$ grep callee.*increment READELF
[3115] callee.increment: 5 [global, function]
...

The regression was fixed by commit 67e83a0dee ("Fix regression in
c-linkage-name.exp with gdb index"), which got us again:
...
$ grep callee.*increment READELF
[1568] callee__increment: 5 [global, function]
[3115] callee.increment: 5 [global, function]
...

The commit however did not claim that particular PR.  A subsequent commit,
commit 5fea979432 ("Improve Ada support in .gdb_index") did claim to fix it,
together with commit dd05fc7071 ("Change .gdb_index de-duplication
implementation").

The commit 5fea979432 contained the following addition in write_cooked_index:
...
+      if (entry->per_cu->lang () == language_ada)
+	{
+	  /* We want to ensure that the Ada main function's name
+	     appears verbatim in the index.  However, this name will
+	     be of the form "_ada_mumble", and will be rewritten by
+	     ada_decode.  So, recognize it specially here and add it
+	     to the index by hand.  */
+	  if (entry->tag == DW_TAG_subprogram
+	      && strcmp (main_for_ada, name) == 0)
+	    {
+	      /* Leave it alone.  */
+	    }
+	  else
+	    {
+	      /* In order for the index to work when read back into
+		 gdb, it has to use the encoded name, with any
+		 suffixes stripped.  */
+	      std::string encoded = ada_encode (name, false);
+	      name = obstack_strdup (&symtab->m_string_obstack,
+				     encoded.c_str ());
+	    }
+	}
...

The code contains some special handling related to the Ada main function, so
let's look at that one: foo.  Before commit 67e83a0dee we have:
...
$ grep foo.*function READELF
[3733] foo: 7 [global, function]
...
and after:
...
$ grep foo.*function READELF
[2738] _ada_foo: 7 [global, function]
[3733] foo: 7 [global, function]
...
so that looks identical to the callee.increment case.

At commit 5fea979432, we have slightly different index numbers:
...
$ grep foo.*function READELF
[1658] foo: 7 [global, function]
[2738] _ada_foo: 7 [global, function]
...
but otherwise the same result.

If we disable the special handling of the Ada main function like so:
...
-	  if (entry->tag == DW_TAG_subprogram
+	  if (false && entry->tag == DW_TAG_subprogram
...
we still have the exact same result because:
...
(gdb) p main_for_ada
$1 = 0x352e6a0 "_ada_foo"
...
and ada_encode ("_ada_foo", false) == "_ada_foo".

The comment seems to be copied from debug_names::insert, which does indeed use
ada_decode, while the code in write_cooked_index uses ada_encode instead.

Remove the superfluous special handling of Ada main in write_cooked_index.

Tested on x86_64-linux, with target boards unix and cc-with-gdb-index.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/dwarf2/index-write.c

@@ -1128,8 +1128,6 @@ write_cooked_index (cooked_index *table,
 		    const cu_index_map &cu_index_htab,
 		    struct mapped_symtab *symtab)
 {
-  const char *main_for_ada = main_name ();
-
   for (const cooked_index_entry *entry : table->all_entries ())
     {
       const auto it = cu_index_htab.find (entry->per_cu);
@@ -1139,25 +1137,12 @@ write_cooked_index (cooked_index *table,
 
       if (entry->per_cu->lang () == language_ada)
 	{
-	  /* We want to ensure that the Ada main function's name
-	     appears verbatim in the index.  However, this name will
-	     be of the form "_ada_mumble", and will be rewritten by
-	     ada_decode.  So, recognize it specially here and add it
-	     to the index by hand.  */
-	  if (entry->tag == DW_TAG_subprogram
-	      && strcmp (main_for_ada, name) == 0)
-	    {
-	      /* Leave it alone.  */
-	    }
-	  else
-	    {
-	      /* In order for the index to work when read back into
-		 gdb, it has to use the encoded name, with any
-		 suffixes stripped.  */
-	      std::string encoded = ada_encode (name, false);
-	      name = obstack_strdup (&symtab->m_string_obstack,
-				     encoded.c_str ());
-	    }
+	  /* In order for the index to work when read back into
+	     gdb, it has to use the encoded name, with any
+	     suffixes stripped.  */
+	  std::string encoded = ada_encode (name, false);
+	  name = obstack_strdup (&symtab->m_string_obstack,
+				 encoded.c_str ());
 	}
       else if (entry->per_cu->lang () == language_cplus
 	       && (entry->flags & IS_LINKAGE) != 0)